[OE-core] gcc 6.1+ and isystem

[Jack Mitchell]

Some of the headers shipped with gcc 6.1 and above now use #include_next 
to try to and do clever things with munging system header files. Our 
injection of isystem into the build at 'meta/conf/bitbake.conf' seems to 
be causing some programs to fail to compile. A full explanation can be 
found at [1], a bug report from GCC specifying that it should only be 
used in extreme cases at [2].

Since we seem to be adding -isystem unconditionally to BUILD_CFLAGS from 
bitbake, and that the default behavior has now changed should this be 
revisited? I'll admit that I am no where near experienced enough with 
GCC and friends internals to make a call on this one, I'm just looking 
for some input.

Regards,
Jack.

[1] 
http://stackoverflow.com/questions/37218953/isystem-on-a-system-include-directory-causes-errors
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129